[DNT] Guard parent for web widgets during WCVA's teardown.

https://crrev.com/c/5900113 was incomplete, in that when WCVA's dtor is
called, parent_for_web_widget is then reset to null.

It's hard to write a proper test in //content because
Shell::DestroyContents will mark the WebContents invisible first. When
the WebContents becomes invisible, the animation manager is already
destroyed.

Bug: 370494381
Change-Id: I9224dbac027b869927263f08b5b8277c7f588235
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5912774
Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>
Commit-Queue: William Liu <liuwilliam@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1365048}